Field of the Invention
The present invention concerns a drawing and/or straightening unit, applicable to feed to a work machine oblong metal products, such as bars, round pieces or metal wire, for example of the type used to make reinforcement for the building trade. In particular, the drawing and/or straightening unit according to the present invention is applied, preferentially but not only, to machines that work simultaneously at least two bars, round pieces or metal wires at a time, feeding them in a substantially uniform, coordinated and simultaneous manner.
Description of Related Art
Bending/shaping machines are known, also called stirrup machines, that are fed with oblong metal products, such as metal wire from a roll, iron round pieces, pre-sheared metal wire or bars, to make reinforcement stirrups for the building trade.
The machines are generally fed with two or more metal products at a time, in order to optimize the productivity of the machine.
Both metal products fed from a roll, and also pre-sheared bars, are generally obtained at the end of the hot rolling cycle and have on the outside a plurality of ribs, so as to improve the conditions under which they grip the concrete, during use.
Since the metal products are made by rolling, and also due to the presence of the ribs, the section of the metal products is not perfectly circular, and they have an oval section characterized by a bigger axis, in correspondence with the ribs, and a smaller axis staggered by about 90° from the bigger axis and smaller by a few millimeters.
It is also known that the work machines, generally at the head, have a drawing and/or straightening unit to feed the metal products to the operating stations of the machine.
The known drawing and/or straightening unit consists of a plurality of rollers opposite with respect to the axis of feed of the metal products, in order to draw them and/or stretch them. Known drawing and/or straightening units normally comprise a driven roller provided with one or more circular throats, in which the metal products to be fed are disposed, and one or more contrasting rollers, opposite the driven roller with respect to the metal products.
The contrasting rollers are not constrained to each other, and each of them exerts a determinate pressure in the direction of the driven roller, in correspondence with a relative circular throat.
The contrasting action is intended to ensure a sufficient friction between the metal product and the driven roller, limiting possible slipping of the metal product in the circular throats.
It is also known that the contrasting rollers can be selectively made to approach the driven roller, depending on the nominal diameter of the metal products being worked.
Moreover, known contrasting rollers are normally cushioned by means of respective elastic elements, for example cup springs, pre-loaded to absorb vibrations, or to compensate, with the pressure exerted, slight variations in the diameter of the metal products as they move.
The approach movement and the pre-loading of the elastic elements in the contrasting rollers are pre-defined and pre-adjusted, before the start of the steps to feed the metal product, depending on the sizes and type of metal product fed.
During the normal feed of known metal products, in particular but not only when fed from a roll, the metal products tend to rotate on themselves, thus varying, during feed, their angular orientation with respect to their longitudinal axis.
This angular variation can lead to a limit condition in which a metal product is orientated with its bigger axis aligned between the driven roller and the respective contrasting roller, whereas another metal product is orientated with its smaller axis aligned between the two rollers, respectively driven roller and contrasting roller.
In order to guarantee drawing even in this limit condition, the adjustment of the approach movement and pre-loading of the elastic elements is carried out empirically in an intermediate condition, presumably valid for both limit conditions of angular orientation of the metal products.
In practice, however, the metal product orientated on the smaller axis is not subjected to sufficient contrast, and slips partly in the relative circular throat, whereas the metal product orientated on the bigger axis is over-contrasted.
The different contrasting action of the contrasting rollers on the individual metal products causes different speeds of feed of the metal products and the relative sliding thereof during feed. Consequently, errors occur in the feed to the operating stations of the machine, and there is a risk of loops forming and the metal products stretching.
In the state of the art, in fact, there is a frequent risk that stirrups may form that have different sizes and that do not correspond to the design data set.
Moreover, the excessive contrast on one of the metal products causes an increase in wear on the circular throats of the driven roller which, with time, lose their efficient guide function, for a correct feed of the metal products.
Purpose of the present invention is to achieve a drawing and/or straightening unit that allows to obviate the disadvantages of the state of the art in a simple and effective way, allowing a simultaneous and coordinated feed of several metal products at a time, substantially whatever the angular orientation of each of them.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.